Steam generators of pressurized water nuclear reactors contain a bank of a very large number of tubes, generally several thousand tubes, bent in U-shape so as to have two straight branches and a bend of semi-circular shape connecting the two straight branches. The ends of the straight parts of the tubes are fixed in a tube plate and form the bottom part of the bank. The juxtaposed bends of the tubes constitute the top or chignon of the bank of tubes.
The tubes of the bank are not identical to one another, since the radius of curvature of the bend varies with the position of the tube in the bank. As a general rule, the tubes situated in the central part of the bank have a small radius of curvature, and the tubes situated on the periphery have a large radius of curvature.
These tubes are disposed in vertical planes parallel to one another and separated by a substantially constant distance corresponding to the distance separating two successive rows of holes in the tube plate.
The straight parts of the tubes are supported in the transverse direction by spacer plates having apertures substantially reproducing the network of through holes in the tube plate. The spacer plates are fixed at their periphery on a cylindrical casing surrounding the bank of tubes.
The top part of the tubes in the bank, comprising successive rows of bends separated by a substantially constant distance, is provided with spacer and wedging members disposed in the spaces formed between two successive rows of tubes.
Not only the spacer plates supporting the straight part of the tubes but also the devices supporting the bends in the chignon must permit effective support of the tubes without hindering the circulation of the steam generator feed water in contact with the outer surface of the tubes of the bank. The pressurized primary cooling water of the nuclear reactor circulates inside the tubes of the steam generator tube bank and permits the heating and vaporization of the feed water circulating in contact with the outer surface of the tubes.
The circulation at high speed of currents of fluid in contact with the tubes of the bank gives rise to stresses which may lead to the setting up of vibrations in these tubes when the steam generator is in operation. This may result in wear or damage to the tubes in the course of a long period of utilization of the steam generator.
The spacer plates provide effective support of the tubes in transverse directions, so that the straight part of the tube bank is not greatly subject to vibrations.
On the other hand, the design of effective anti-vibratory devices in the top or chignon of the tube bank presents a difficult problem.
For the purpose of supporting the bends in the chignon of the tube bank, it has been proposed to use antivibratory bars which are disposed in the spaces between the rows of tubes, in order to provide a wedging action.
These antivibratory bars must have a thickness less than the width of the spaces separating two successive rows of tubes, in order to enable them to be inserted between the rows in the chignon without difficulty and without displacing or damaging the bends between which the antivibratory bars are inserted.
Furthermore, if manufacturing tolerances of the tubes and antivibratory bars are taken into account, the residual installation clearance for the bars in the chignon is still further increased.
The presence of a relatively large clearance between bars and the tubes results in vibration during operation, which is accompanied by shocks and friction which give rise to wear and damage to the tubes.
Excessive wear on a tube in the bank makes it necessary for it to be put out of action, for example by closing its two ends, during a period when the nuclear reactor is shut down. Such operations for the closing of the tubes are lengthy and expensive and result in a reduction of the exchange surface of the steam generator and of the steam pressure generated.
It has therefore been attempted to use more effective anti-vibratory bars for wedging the tubes in the chignon.
It has, for example, been proposed to use anti-vibratory bars which are expansible in the direction of their thickness and which comprise a pair of adjustable rods connected together and adapted to be moved relative to one another between a first position, in which the anti-vibratory bar has a thickness less than the space between two successive rows of tubes, and a second position in which the bar has a thickness greater than that corresponding to the first position of the adjustable rods.
According to French Patent Application No. 2,558,933, each pair of adjustable rods constituting an anti-vibratory bar comprises complementary rod sections having oblique surface zones placed face to face and forming longitudinal ramps whose relative displacement permits transverse expansion of the bar.
Devices of this kind are complex in construction and require the use of operating means for moving one of the rod sections comprising ramps in relation to the other.
The anti-vibratory bars can be inserted relatively easily between the rows of tubes in the unexpanded state, but the bars must then be expanded in position in the chignon by operating the rod displacement device, which may consist, for example, of a screw and nut system. This operation requires intervention in a zone of the steam generator accessible only with difficulty, where there may in addition be ionizing radiation if the intervention is required in a steam generator which has already been in operation. This difficulty exists in all cases where it is desired to replace non-expansible anti-vibratory bars by mechanically operated expansible bars according to the conventional technique.
Furthermore, these mechanically operated bars have the disadvantage of requiring relative longitudinal movement of one part of the bar in relation to the other while the bars are in position in the chignon.
In the case of an installation operating at a constant or practically constant working temperature, generally speaking, no process which is both effective and simple to carry out has been known to date for the anti-vibratory wedging of component parts of the installation which during operation are subjected to stresses capable of causing them to vibrate and which are separated by a substantially constant space over the width of which the parts are vibratable.